Power transistors



Feb. 9, 1960 all? f A. HERLET POWER TRANSISTORS Filed Nov. 25, 1958 @QM.//////////M FigAb Fig.5

POWER TRANSISTORS Adolf Herlet, Pret'zfeld/Ofr., Schloss, Germany,assigner to Siemens-Schuckertwerke Aktiengesellschaft, Berlin-Sie-Mensstadt, Germany Application November 25, 1958, Serial No. 776,323Claims priority, application Germany November 30, 1957 4 Claims. (Cl.317-235) My invention relates to power transistors consisting of amonocrystalline silicon plate with at least two heavily doped regions ofone conductance type, namely an emitter region and a collector region,and an intermediate, less heavily doped base region of the otherconductance type. Such semiconductor devices of silicon or germanium aregenerally known as p-n-p or n-p-n transistors. In known silicon powertransistors, the collector electrode occupies `the major portion of oneof the at sides of the silicon plate, whereas the emitter electrode andthe base electrode cover respective areas on the other flat side inspaced relation to each other, usually so that the emitter is circularand the base forms an annular strip around the emitter.

I t is an object of my invention to increase the efficacy and currentcarrying capacity of such power transistors `for any given size; and itis another object to achieve this without increasing the manufacturingcost beyond the economical limit of mass production. p

To this end, and in accordance with a feature of my invention, Isubdivide the base electrode into two or more parts spaced from eachother to form an intermediate gap, and locate the emitter electrode inform of a strip in that gap, the emitter strip having a widthsubstantially equal to 2\/ times the diffusion length at high injection,aside from covering a total area of several square millimeterssufiicient for power current purposes. The term highinjection relates tothe operating condition of the semiconductor device as a powertransistor, that is to a chargecarrier injection suicient to raise both(n,p) charge-carrier concentrations in the base region of thesemiconductor 'i crystal considerably above the level of the doping(donor, acceptor) concentration.

The following explanation will aid in understanding the invention. Aportion of the driving voltage applied between the emitter and base of atransistor is effective between the p-n junction proper which is locatedbetween emitter region and base region. Another portion of the drivingvoltage is consumed within the base region for laterally driving on thebase current. `In principle, such lateral'voltage drop is alwayspresent; but it becomes appreciable only if the base region is subjectedto the condition of high injection, that is when both carrierconcentrations are raised far above the doping concentration. Thiscondition obtains, as a rule, in power transistors operating with highcollector currents. Hence, at such current loads the voltage drop due tothe lateral ow of base current is so large as to appreciably reduce thevoltage drop at the p-n junction. Considering diierent current now pathsin the semiconductor, such reduction in junction voltage is the greaterthe more the origin point of the current path is remote from the edge ofthe emitter.

As a result, the carrier injection from the emitter decreases inexponential relation to the residual local voltage at the p-n junction.

lead to my invention, that the interior area elements of States Patent OBased upon these phenomena 1s the recogmtion, which 2,924,760 iatentedFeb. 9, M960 Mice the centrally located emitter in the known powertransistors contribute less to the emitter current than the marginalarea elements. It has been found that in a silicon n-p-n transistor ofthe known type described above, the circular, centrallylocated emitterarea is effective substantially only at its outer marginal zone having awidth of V Lp if the radius of the circular emitter is large relative tothe amount of \/2 Lp, Lp being the ditusion length in the base region athigh injection. Thisnding is based upon the simplified assumption that,with a su'iciently heavy doping of the emitter region and a slightspacing between emitter and base, only the volume recombination is to betaken into account for determining the base current, so that the emittercurrent can be considered as effective and the surface recombination beneglected. In contrast to such limited utilization of the emitter areain the known power transistors, the invention, by virtue of thesubdivision ofthe base electrode and the provision of a large-areaemitter in form of a strip about 2\/2 Lp wide in the gap of the baseelectrode, greatly increases the efcacy and thus permits a highercurrent loading of a power transistor of given size. This will be morefully explained below with reference to the drawing in which:

Figs. la and lb are a cross section and a sectional plan view of atransistor according to the prior art, the sections being along thelines la-Ia and lb-Ib, respectively.

Figs. 2a and 2b are a cross section and a sectional plan View of anembodiment of a power transistor according to the invention, thesections being along lines IIa-Ila and Hbf-1lb, respectively.

Figs. 3a and 3b are a cross section and a sectional plan view of anotherembodiment of a power transistor according to the invention, the sectionlines being denoted by Illa-Illa and lllb-IIlb.

Figs. 4a and 4b are a plan view and a side view respectively of a thirdtransistor according to the invention; and

Fig. 5 is a plan view of still another embodiment of the invention.

All illustrations are on enlarged scale.

The example of a power transistor according to the prior art illustratedin Figs. 1a and 1b comprises a silicon plate S ofv p-type conductancewhose area is of square shape has an edge length of 8 mm., for example.The bottom side of the silicon plate S, whose original thickness was0.08 mm., is provided with a collector C which nearly covers the entirebottom side. The collector is produced by alloying into the surface zoneof the silicon plate a gold foil containing 1% antimony and having athickness of 0.04 mm. A p-n junction extends over the entire area of thecollector facing the interior of the silicon plate. Also joined with thesilicon plate by alloying is an emitter E of circular shape Whose radiusis denoted by re. The emitter, located on the other ilat side of theplate, is concentrically surrounded by a ringshaped base contact B whichis formed by alloying in a ring-shaped aluminum foil of 0.04 mm. Fortesting purposes, a number of silicon transistors of this type wereproduced with respectively diterent diameters of the emitter area, butwith always the same marginal spacing of the base ring. The emittercurrents were measured, with an ampliiication factor @2:10, independence upon the emitter radius. The results conrmed theabove-mentioned linear dependence of the emitter currents upon theemitter diameter, in contrast to the square-law of increase in emitterarea.

A better utilization of the emitter area and therefore also of thesemiconductor area, was obtained with the transistor type according tothe invention illustrated in Figs. 2a and 2b. ln this power transistorthe area of the emitter had the same size as in the above-describedtran- 3 sistor of Figs la and-1b, and the production ofthe electrodes byalloyingmthemI together with the silicon body, as well as the materialsused were also the same. However, in the transistor according to Figs.2a, 2b, the emitter area is formed by a vring-'shaped strip E, and thebase electrode is subdivided into a base ring "B1 surrounding theemitter ring and a base electrode B2 of circular shape concentricallylocated within the emitted ring. The utilizable marginal area oftheemitter ring E is twice as large as with the power transistor accordingto Figs. la and lfb, so Vthat under otherwise equal 'conditions themagnitude ofthe emitter current is doubled.

In the .foregoing oomparisoniof the two. transistors it is assumed thatthe median radius re according to Figs. 2a and' 2b isequal,,totheradiusrrevof the circular en litter area accordingto Figs.la and lb, and that thewidthof the emitter ring 4Erin Figs. 2a,2b isYequal toene-half of that radius. namely 0.5 la In .accordance ,withnyinvention, the Vvemitter `strip according tonFigs. y2 a, 2b is given eWidth as Close as possible tothe-mest .favorable value of 2\/2 Lp. Ifthe width is made considerably larger, the utilization of the emitterarea is too slight, and with a considerably smaller'width for a givenemitter `area the length of the emitter .strip becomes too large sothat, for a given median radius,`the numberoflenjlitter rings would haveto be increasedV with' result of increasing the manufacturing costwithout improving the area utilization toy aporresponding degree.

ln commercial manufacture, satisfactory power tran- Sistors according toFigs. 2a and 2b have been produced by alloying lwith the silicon discrespective emitterustrips of 0, 8 and l mm. widthin accordancewith theillustrated ring pattern; and a spacing between 0.05 and 0.1 wasprovided between the emitter ring E and the simultaneously alloyed twoportions yB1 and yB2 of the base electrode. With these widths, theabove-,mentioned ring Shaped foils of gold and aluminum should beconveniently manipulated prior to the alloying process proper. One oflthe alloying methods used consisted in embedding the silicon platetogether with the shaped foils yfor the base, emitter and collector ingraphite powder inproper` position relative to one another, and thenheatingV the Ventire assembly to the alloying temperature. Thiskmethlodis more fully described in the copending application of R. Emeis, SerialNo. 637,029 tiled January 29, 1257, and assigned to the assignee of thepresent invention. rThe ring-shaped electrodes thus alloyed together,with the ,silicon disc readily pertutedettaching the connecting leadsof wire or metal strips by soldering. The above-mentioned width of theemitter strip, amonnting Jto 0.8 or l mm., Closely approaehes. theoptimum for bestutilization of the emitter area, becausethe diffusionlength Lp at Vhigh injection may attain-the amount of 0.3 mm. Subsequentchecking is possible by determining the diffusion length Lp in theVknownmanner from the measurableMq-yalue and the thilnessofthebaseelectrode.

The dimensioning of the emittervwidth `according to the invention isalso of advantage if the measured diffusion length Lp has smallervalues, for example 0.2 or 0.1 mm.,`because foil strips of smallerwidthhave very little stiffness and for thatzre'asonA are more` diicultto manufacture and to manipulate, and the attachment of,k th

current supply leads is morediiicnlt. l-"urthermore,A the entire'emitterarea in each case covers only approximately one-third of the originalsemiconductor surface sol that if one goes beyond themost favorablewidth of the emitter strip by the factor 3, only 20% of thetotal area onone side ofthe silicon plate is insufficiently utilized. This iseconomically justiablein comparison withV the otherwise necessaryincrease in manufacturing cost and in some Acases is also' of advantagewith respect to cooling conditions. However, itis also possible, forexample by depositing the electrode metal by vaporizationorelectrolytically, top'roduce emitter strips, and if desiredalso basecontacts, of widthsdown.to approximately 0.4

mm., to which currwent .supplynleadspcan bey attached at downwardly fromthe most favorable value 2\/2 Lp are sometimes advantageous for certainoperating conditions or for the above-mentioned reasons of manufacture.

vln the 'modified power transistor according tofFigs. 3a and 3b, thelbase electrode is further'subdivided, and the emitter electrode is alsosubdivided'so that the illustrated rings form alternately part of theemitter E and part of the base B. The largest emitter ring E issurrounded by a 'still larger base ring, land `a base ring is locatedwithin the smallest emitter ring, so thatbase strips are located on bothsides of each individual emitter strip. As a result the entire marginallength of the composite emitter area is elfectivelyfutilized. Ifdesired, `theinnermost base electrode ring may also consist of 'a fullcircular area.

Power transistors have been lproduced in accordance with such amultiple-ring pattern by alloying into the silicon body S two ring foilsof antimony-containing gold and three aluminum foils, all of l mm.width. These power transistors weretested and were found to be suitable`for up to ,20 amps. output current flowing through emitter andcollector lwhengoperating with 2.5 amps. of base current. ,f Thecollector C was made from a goldvantinllony f oil ofsomewhat largerdiameterthan the silicon disc` S. `This had the elfectthat the alloyformation ,between Si and Au/Sb also extended to the peripheral sideofthe Si disc. The resulting formation of the collector C is apparentfromFig. 3a. It hasthe advantage that ,the external p-n boundary of thecollector-side Ip-n junction is located ,on the lupper -flat surface oflelectrolytically with a gold coating which is then heated to atemperatureof 800 to 900 C. ,to become rmly adhel-"ent".-vTlie'v'carrie'r'plate, thus prepared, can then be firmly joined withtthe' Vtransistor element` by heating both temporarily upto'approximately'40.0'o C. without ascertamany meeting the pejviou'slyfarmed nyef 'structure and Velectrical properties of the 'trans1st'orflIn v'the transistor elementshown'infFigs. 4a and 4b, the emittervandbase electrodes form a straight strip pattern. Akbase contact strip Bis"`located`on` both sides respectively of 'eachofthe'emitter strips E.All strips may'have the width,"forl example 0.'8"toV l mm. The electrodestrips pertaining to ,one and the same electrode are-tobeelectricallyinterfonlriect'ed,` s in all other embbdiments. Theinter'connectonaccording to Fig. 4a is effected' by meanso'f bent metal strips ZEsoldered to all emitter strips, 'and ZB'soldered to all base strips. Thecurrent-supplystip 'ZE may consist of Vcopper which is' rstgalvanically'plated ,with silver, then gold coated,

and ultimately `welded t`o the electrode strips lat Vthe points'of"contact-jengegementf` A similar current-supply strip ZB 'may' frstbecoated with Atin at the points of contact and then be soldered to' theetched aluminum strips B which are vlikewisetiri-coated at'the points ofcontact engagement. Several such supply leads may be provided forfthevbase strips [and particularly also for the emitter strips: becauseA thelatter, as arule, carry higher currents. in the'same lriilaitiier,transistors embodying a ring pattern Y .iZfQiZIilo'r 3a, 3i?,` maybeprovided with S'uclfiJ current-supply `conductors. vThe conductors mayalso be arranged in crosswise or starwise relation to each `other,whereas kin a straight-strip pattern, according to Figs. 4a, 4b, therespective conductors are preferably mounted one longitudinally besidethe other.

The transistor shown in Fig. 5 has its base and emitter electrodearranged in a comb-shaped pattern known as such. The strips pertainingto the emitter E extend int the comb gaps of the base contact area B.The width of the emitter strips E, and preferably also the correspondingstrips of the base B, are to be given a width substantially equal to 2\/Lp in accordance with lthe invention, this width is between 0.1 and 1mm.

While the invention is described above with reference to examples ofn-p-n power transistors, it is analogously applicable to transistors ofthe p-n-p type. In the latter case the value Lp is to be replaced by thediiusion length Ln in the n-conducting base region. The dimensionsapplying to such p-n-p transistors are substantially the same as givenabove for n-p-n transistors.

I claim:

l. A power transistor, comprising a monocrystalline silicon plate havingjoined therewith a base electrode, an emitter and a collector and havingheavily doped regions of one conductance type adjacent to said emitterand collector respectively, and a relatively lightly doped region of theother conductance type connected to said base electrode by anon-rectifying connection, said two heavily doped regions on the onehand and said lightly doped region on the other hand forming respectivep-n junctions at their mutual boundaries, said collector covering amajor portion of one at side of said plate, said base electrode having aplurality of mutually spaced portions covering part of the other side ofsaid plate and forming a gap between each other, said emitter beingstrip-shaped and covering part of said other side in said gap, saidemitter having a total area of several square millimeters for carryingpower current and having a strip width approximately equal to 2\/ timesthe diffusion length at high injection.

2. In a silicon power transistor according to claim 1, said width ofsaid strip-shaped emitter departing from the exact value of 2\/2 timesthe diffusion length at most by the factor three.

3. In a silicon power transistor according to claim 1, said width ofsaid strip-shaped emitter being between 0.4 and 1.0 mm.

4. A silicon power transistor `according to claim 1, comprising aplurality of ring-shaped and cocentric emitter strips, conductivelyconnected with each other, and said base electrode having respectiveportions located around the largest emitter ring and within the smallestemitter ring respectively, said portions being conductively connectedwith each other.

References Cited in the le of this patent UNITED STATES PATENTS

